Protection of mission-critical push-to-talk multimedia broadcast and multicast service subchannel control messages

ABSTRACT

A Mission-Critical Push-To-Talk (MCPTT) node is connected to a group of user equipments (UEs) served by the MCPTT node. The MCPTT node generates a Multimedia Broadcast and Multicast Service (MBMS) Subchannel Control Key (MSCCK), sends first messages to the UEs in unicast, wherein the first messages include the generated MSCCK, generates at least one MBMS subchannel control message, applies integrity protection and/or confidentiality protection to the at least one MBMS subchannel control message with the MSCCK, and sends the at least one MBMS subchannel control message in multicast.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is divisional of U.S. patent application Ser. No.15/591,934 filed on May 10, 2017, which claims the priority to U.S.Provisional Patent Application No. 62/414,890 filed on Oct. 31, 2016,the subject matter of which are hereby incorporated by references.

TECHNICAL FIELD

The disclosed subject matter relates generally to telecommunications.Certain embodiments relate more particularly to Mission-CriticalPush-To-Talk (MCPTT).

BACKGROUND

MCPTT is a 3GPP-related technology that provides half-duplexcommunication between users of a MCPTT group. MCPTT is intended toprovide secure and reliable communication for government and privateactors such as police, fire fighters, and ambulance personnel for whomthe availability of communication is Mission-Critical.

MCPTT enables multicast distribution of media and floor control messagesto User Equipments (UEs) using a Multimedia Broadcast and MulticastService (MBMS) bearer. It also enables direct unicast communication toeach individual UE.

FIG. 1 is a block diagram illustrating an example context in whichmulticast distribution may occur. More specifically, FIG. 1 illustratesan MCPTT on-network architecture 100 implementing MBMS as presented inFIG. 5.2.7-1 of 3GPP TS 23.179 V13.5.0.

Referring to FIG. 1, MCPTT on-network architecture 100 comprises anMCPTT client located in a UE 105, an MCPTT server 110 shown bundled witha Group Communication Service Application Server (GCS AS), and anEvolved Universal Mobile Telecommunications System (UMTS) TerrestrialRadio Access Network (E-UTRAN) 115 located between UE 105 and MCPTTserver 110. E-UTRAN 115 provides communication between UE 105 and MCPTTserver 110 via a unicast path 120 and an MBMS path 125. In general,communication may be conducted between the various features ofarchitecture 100 via interfaces as illustrated in FIG. 1 and describedin 3GPP TS 23.179.

FIG. 2A is a signal diagram illustrating a conventional method 200A forperforming multicast distribution, and FIG. 2B is a flowchartillustrating method 200B, which is a simplified version of the method200A. These methods could be performed e.g. in a context such as thatillustrated in FIG. 1.

Referring to apparatus features in FIG. 2A, three UEs 205A-C compriseMCPTT clients, and they communicate with an MCPTT server 210 throughunicast connections prior to activation of an MBMS bearer. Once the MBMSbearer is activated, the UEs receive multicast messages from MCPTTserver 210.

MCPTT server 210 may transport media and floor control messages ofseveral sessions via one MBMS bearer, and different UEs can participatein one or more of those sessions. For instance, in the example of FIG.2, MCPTT server 210 uses a single MBMS bearer to transport media andfloor control messages for a session X for an MCPTT group G1. MCPTTserver 210 uses unicast bearers to transport media and floor controlmessages for a session Y for an MCPTT group G2, and for a session Z forMCPTT group G3. UE 205A participates in session X, UE 205B participatesin session Y and UE 205C participates in session X and Z.

Referring to signaling features in FIG. 2A, in steps 0 a, 0 b, 0 c and 0d, media and floor control messages of sessions X, Y and Z aredistributed between UEs 205A-C and MCPTT server 210. As indicated above,UE 205A participates in session X for MCPTT group G1, UE B participatesin session Y for MCPTT group G2 and UE C participates in session X forMCPTT group G1 and Z for MCPTT group G3.

In step 1, MCPTT server 210 activates an MBMS bearer. Thereafter, insteps 2 a, 2 b and 2 c, MCPTT server 210 announces the MBMS bearer,using unicast Session Initiation Protocol (SIP) signaling, to the UEsusing a SIP message request sent in accord with 3GPP TS 24.379 subclause14.2.2.2. The SIP signaling includes media parameters and internetprotocol (IP) address and port for the MBMS general purpose MBMSsubchannel, media parameters for transport of media and floor controlmessages (excluding destination multicast IP addresses and ports) viaMBMS bearer, and Temporary Mobile Group Identity (TMGI).

In steps 3 a, 3 b, 3 c, when each UE detects that the MBMS beareridentified by the TMGI is available, each UE informs the MCPTT server210, using unicast SIP signaling, about the availability of the MBMSbearer in the location of the UE by sending a SIP message requestcontaining an MBMS bearer listening status set to “listening” in accordwith 3GPP TS 24.379 subclause 14.3.3.2. Then, the UE starts receivingmessages on the MBMS general purpose MBMS subchannel of the MBMS bearer.

In step 4, MCPTT server 210 decides to send media of session X via theactivated MBMS bearer. Thereafter, in step 5, MCPTT server 210 sends aMap Group To Bearer message via MBMS bearer and indicates that sessionof MCPTT Group G1 will be sent via MBMS bearer using particular MBMSsubchannels with indicated IP addresses and indicated ports for mediaand floor control messages.

In steps 6 a and 6 c, based on reception of the “Map Group To Bearer”message, UEs 205A and 205C start listening for media and floor controlmessages received via the MBMS bearer on the IP addresses and ports asindicated in the Map Group To Bearer message. UE 205B receives the “MapGroup To Bearer” message too, but because it relates to MCPTT Group G1where UE 205B does not participate, UE 205B ignores the Map Group ToBearer message.

In steps 7 a-e, media and floor control messages of session X aredistributed between UE 205A, UE 205C and MCPTT server 210. In uplink,the media and floor control messages still use unicast. In downlink, themedia and floor control messages interesting to both UE A and UE C aresent via MBMS bearer on the IP addresses and ports as indicated in theMap Group To Bearer message in step 6 a, 6 c. In downlink, the floorcontrol messages interesting to UE A only and UE C only are sent viaunicast.

In steps 7 x and 7 y, media and floor control messages of sessions Y andZ are distributed between UE 205B, UE 205C and MCPTT server 210. Theyare unchanged (i.e. they still use unicast transport) as MCPTT Group G2and MCPTT Group G3 were not indicated in the “Map Group To Bearer”message in steps 6 a and 6 c.

Referring to FIG. 2B, method 200B comprises steps S205-S230, which aresimplified or distilled versions of the steps in method 200A. In stepS205, MCPTT server 210 activates an MBMS bearer. Thereafter, in stepS210, MCPTT server 210 announces the MBMS bearer, using unicast SessionInitiation Protocol (SIP) signaling, to UEs 205A-C using a SIP MESSAGErequest sent in accord with 3GPP TS 24.379 subclause 14.2.2.2.

In step S215, each UE informs the MCPTT server, using unicast SIPsignaling, about availability of the MBMS bearer in location of the UE.It does so by sending a SIP message request containing MBMS bearerlistening status in accord with 3GPP TS 24.379 subclause 14.3.3.2.

In step S220, when a sufficient number of UEs report availability of theMBMS bearer at a given location, MCPTT server sends a “Map Group ToBearer” message via the MBMS bearer (steps 4 & 5 of FIG. 2A), startssending media and (some types of) floor control messages using MBMSbearer (i.e. using multicast), and stops sending media and (some typesof) floor control messages using unicast to the UEs which informed theMCPTT server about availability of the MBMS bearer. When a UE receivesthis message, the UE starts receiving and rendering the media and floorcontrol messages via the MBMS bearer (i.e. using multicast).

In step S225, as each UE moves, it may move to a location where the MBMSbearer is no longer available. In such case, the UE informs the MCPTTserver, using unicast SIP signaling, about non-availability of the MBMSbearer in location of the UE.

In step S230, when the amount of UEs reporting availability of the MBMSbearer at a given location gets low, MCPTT server sends an “Unmap GroupTo Bearer” message via the MBMS bearer, stops sending media and (sometypes of) floor control messages using MBMS bearer (i.e. usingmulticast), and starts sending media and floor control messages usingunicast to all the UEs. The UEs starts receiving the media and floorcontrol messages using unicast.

In the methods illustrated in FIGS. 2A and 2B, the “Map Group To Bearer”message and “Unmap Group To Bearer” message each contain MCPTT Groupidentifier (ID), which is supposed to not be disclosed to otherunauthorized UEs in accord with TS 33.179. These constraints aredescribed in further detail in 3GPP TS 24.379 subclause 14 and 3GPP TS24.380 subclause 4.1.3.

SUMMARY

In some embodiments of the disclosed subject matter, a UE comprises anMCPTT client equipped to be connected to a MCPTT node that serves theUE. The UE comprises processing circuitry, memory, and transceivercircuitry collectively configured to receive a first message from theMCPTT node in unicast when activating an MBMS bearer, wherein the firstmessage comprises an MBMS Subchannel Control Key (MSCCK), identify theMSCCK from the message, receive an MBMS subchannel control message fromthe MCPTT node, and apply integrity protection and/or confidentialityprotection to the MBMS subchannel control message using the MSCCK.

In certain related embodiments, decrypting and/or integrity checking theMBMS subchannel control message using the MSCCK comprises generating amessage authentication code (MAC) value using the at least one MBMSsubchannel control message and the MSCCK, and comparing the generatedMAC value to a MAC value appended to the at least one MBMS subchannelcontrol message.

In certain related embodiments, decrypting and/or integrity checking theMBMS subchannel control message using the MSCCK comprises decrypting theMBMS subchannel control message using the MSCCK.

In certain related embodiments, the at least one MBMS subchannel controlmessage comprises a Map Group To Bearer message.

In certain related embodiments, the at least one MBMS subchannel controlmessage comprises an Unmap Group To Bearer message.

In some embodiments of the disclosed subject matter, a method isprovided for operating a user equipment (UE) comprising aMission-Critical Push-To-Talk (MCPTT) node equipped to be connected to aMCPTT node that serves the UE. The method comprises receiving a firstmessage from the MCPTT node in unicast when activating a MultimediaBroadcast and Multicast Service (MBMS) bearer, wherein the first messagecomprises a MBMS Subchannel Control Key (MSCCK), identifying the MSCCKfrom the message, receiving an MBMS subchannel control message from theMCPTT node, and applying integrity protection and/or confidentialityprotection to the MBMS subchannel control message using the MSCCK.

In certain related embodiments, decrypting and/or integrity checking theMBMS subchannel control message using the MSCCK comprises generating amessage authentication code (MAC) value using the at least one MBMSsubchannel control message and the MSCCK, and comparing the generatedMAC value to a MAC value appended to the at least one MBMS subchannelcontrol message.

In certain related embodiments, decrypting and/or integrity checking theMBMS subchannel control message using the MSCCK comprises decrypting theMBMS subchannel control message using the MSCCK.

In certain related embodiments, the at least one MBMS subchannel controlmessage comprises a Map Group To Bearer message.

In certain related embodiments, the at least one MBMS subchannel controlmessage comprises an Unmap Group To Bearer message.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate selected embodiments of the disclosed subjectmatter. In the drawings, like reference labels denote like features.

FIG. 1 is a block diagram illustrating an example context in whichmulticast distribution may occur.

FIGS. 2A-1 and 2A-2 show a signal diagram illustrating a conventionalmethod of performing multicast distribution.

FIG. 2B is a flowchart illustrating a simplified version of the methodillustrated in FIG. 2A.

FIG. 3A is a flowchart illustrating a method of operating an MCPTT node(e.g., an MCPTT server) according to an embodiment of the disclosedsubject matter.

FIG. 3B is a flowchart illustrating a method of operating a UE accordingto an embodiment of the disclosed subject matter.

FIGS. 4A and 4B show a signal diagram illustrating a method ofperforming multicast distribution according to an embodiment of thedisclosed subject matter.

FIGS. 5A and 5B show a signal diagram illustrating a method ofperforming multicast distribution according to another embodiment of thedisclosed subject matter.

FIG. 6 is a block diagram illustrating an MCPTT node according to anembodiment of the disclosed subject matter.

FIG. 7 is a block diagram illustrating a UE according to an embodimentof the disclosed subject matter.

FIG. 8 is a block diagram illustrating a wireless communication networknode according to an embodiment of the disclosed subject matter.

FIG. 9 is a block diagram illustrating a radio device according to anembodiment of the disclosed subject matter.

DETAILED DESCRIPTION

The following description presents various embodiments of the disclosedsubject matter. These embodiments are presented as teaching examples andare not to be construed as limiting the scope of the disclosed subjectmatter. For example, certain details of the described embodiments may bemodified, omitted, or expanded upon without departing from the scope ofthe disclosed subject matter.

Certain embodiments are presented in recognition of shortcomingsassociated with conventional techniques and technologies, such as thefollowing examples.

In conventional systems, the (1) “Unman Group To Bearer” message and (2)“Map Group To Bearer” message (and possibly other MBMS subchannelcontrol messages e.g. as specified in 3GPP TS 24.380 subclause 8.4) arenot protected.

Protection of such messages is desirable for the following reasons.First, at least two types of the currently specified messages (e.g. 1and 2) include MCPTT Group IDs which are classified as sensitive dataand to which confidentiality protection may be applied. Therefore,confidentiality protection is needed.

Second, an attacker replaying or even constructing fake messages candisrupt a call by telling MCPTT UEs to start receiving and rendering themedia and floor control messages via the MBMS bearer while no mediaand/or no floor control messages are sent. This is a denial of serviceattack. Therefore, integrity protection is needed as well. However, 3GPPTS 33.179 does not specify protection for this purpose, which is theprotection of MBMS subchannel control messages.

3GPP TS 33.179 defines two keys for the protection of group callsessions. These keys are the Group Management Key (GMK) and themulticast key floor control (MKFC). The GMK is used for the protectionof the media and the MKFC is used for the protection of the multicastfloor control from the MCPTT server to the MCPTT UE. Similarly, otherkeys not related to group calls are dedicated to other purposes. Forexample, the Private Call Key (PCK) is used to protect private calls,the Call Service Key (CSK) is used to protect signaling between MCPTT UEand MCPTT domains, etc.

Because MBMS subchannel control messages are distributed over MBMSbearers, they cannot be protected by MCPTT UE-specific keys such as thePCK or the CSK. Consequently, they require a different solution.

Certain embodiments provide potential benefits compared to conventionaltechniques and technologies, such as the following examples. In certainembodiments, the “Unmap Group To Bearer” message and “Map Group ToBearer” message as well as other MBMS subchannel control messages (e.g.as specified in 3GPP TS 24.380 subclause 8.4), are integrity andconfidentiality protected and thus cannot be spoofed. This provides aMCPTT system that has a higher threshold for denial of service attacks.

In certain embodiments described below, protection is provided for the“Unmap Group To Bearer” message, “Map Group To Bearer” message, andother MBMS subchannel control messages (e.g. as specified in 3GPP TS24.380 subclause 8.4).

FIG. 3A is a flowchart illustrating a method 300A of operating an MCPTTnode according to an embodiment of the disclosed subject matter, andFIG. 3B is a flowchart illustrating a method 300B of operating a UEaccording to an embodiment of the disclosed subject matter. In somecontexts, the methods of FIGS. 3A and 3B may be performed in coordinatedfashion. For instance, an MCPTT node may perform certain parts of method300A by communicating with one or more UEs performing method 300B, andvice versa.

The method of FIG. 3A may be performed by an MCPTT node connected to agroup of UEs, and the method of FIG. 3B may be performed by a UEconnected to an MCPTT node. In this context, the MCPTT may be any typeof wireless communication network node capable of implementing theindicated functionality, and the UE may be any type of radio nodecapable of implementing the indicated functionality.

Referring to FIG. 3A, the method 300A comprises generating a MultimediaBroadcast and Multicast Service (MBMS) Subchannel Control Key (MSCCK)(S305), sending first messages to the UEs in unicast, wherein the firstmessages contain the generated MSCCK (S310), generating MBMS subchannelcontrol messages (S315), hashing and/or encrypting the MBMS subchannelcontrol messages with the MSCCK (S320), and sending the MBMS subchannelcontrol messages in multicast (e.g. broadcast) (S325).

The term “multicast”, as used in this context, may refer to anytransmission intended for multiple receiving parties, which of courseincludes broadcast transmissions. In this example, the MBMS subchannelcontrol messages may be sent to the group of UEs, although they may notnecessarily reach all UEs served by the MCPTT node, and the MBMSsubchannel control messages may be received by other UEs located in anarea where the MBMS bearer is available. In a typical scenario, the MBMSsubchannel control message is sent towards a multicast address and port,using an MBMS bearer. The multicast address and port and the TMGI aregenerally announced to the UEs in the first messages sent to the UEs inunicast.

The term “hashing”, as used in this context, may refer to a process thatgenerates a MAC, or keyed hash, using the at least one MBMS subchannelcontrol message and the MSCCK, and then and appends the resulting MACvalue to the at least one MBMS subchannel control message.

Referring to FIG. 3B, the method 300B comprises receiving a firstmessage from the MCPTT node in unicast, wherein the first messageincludes a Multimedia Broadcast and Multicast Service (MBMS) SubchannelControl Key (MSCCK) (S330), identifying the MSCCK from the message(S335), receiving an MBMS subchannel control message from the MCPTT node(S340), and hashing and/or decrypting the MBMS subchannel controlmessage using the MSCCK (S345).

In certain related embodiments, an MCPTT node or UE may be configured toimplement method 300A or 300B. For instance, in some embodiments, anMCPTT node configured to perform method 300A is equipped to be connectedto a group of UEs to be served by the MCPTT node. The MCPTT may comprisekey management and a Push-To-Talk (PTT) message generator (i.e., keymanagement and PTT message generation functionalities, including anyrelevant hardware and/or software features). The key management isadapted to generate MSCCKs for hashing and/or encryption of MBMSsubchannel control messages. The PTT message generator is adapted togenerate and send first messages to said UEs containing said MSCCKs inunicast. The PTT message generator is further adapted to generate MBMSsubchannel control messages, hash and/or encrypt them using said MSCCKs,and send them in multicast to the group of UEs.

In some other embodiments, a UE configured to perform method 300B isequipped to be connected to a serving MCPTT node. The UE ischaracterized by key management and a PTT message handler (i.e., keymanagement and PTT message handling functionalities, including anyrelevant hardware and/or software features). The key management isadapted to extract the MSCCK from said first message. The PTT messagehandler is adapted to receive a first message in unicast from a servingMCPTT node, containing a MSCCK for hashing and/or decryption of MBMSsubchannel control messages. The PTT message handler is further adaptedto receive MBMS subchannel control messages in multicast and to hash andor decrypt them using said MSCCK.

FIG. 4 is a signal diagram illustrating a method 400 of performingmulticast distribution according to an embodiment of the disclosedsubject matter. In the illustrated example, the method is performed in asystem comprising a plurality of UEs 405A-C, each comprising an MCPTTclient, and an MCPTT server 410, which may be implemented in a networknode, for ins tnace. The method of FIG. 4 is similar to the method ofFIG. 2, with differences that will be apparent from the description thatfollows.

In the method of FIG. 4, when MCPTT server 410 activates an MBMS bearer,it generates an MSCCK. Thereafter, MCPTT server 410 informs a UE (e.g.,any of UEs 405A-C) of the existence of the MBMS bearer, using unicastSIP signaling, ant it provides the MSCCK to the UE with the SIPsignaling. In some instances, MCPTT 410 server can also provide a newMSCCK to the UE in a standalone SIP message.

MCPTT server 410 uses the MSCCK key(s) to hash (integrity protection)and/or encrypt (confidentiality protection) MBMS subchannel controlmessages.

When the UE receives a “Unman Group To Bearer” message or a “Map GroupTo Bearer” message or other MBMS subchannel control messages (e.g. asspecified in 3GPP TS 24.380 subclause 8.4) via an MBMS bearer, the UEchecks the integrity and confidentiality protection of the message usingthe MSCCK received earlier using unicast SIP signaling.

An integrity check, in this context, includes checking if a senderidentifier (ID) and message content are genuine by checking one or morehash values of the message with the MSCCK. A confidentiality check, inthis context, includes decryption of a message with the MSCCK.

Referring to FIG. 4, the following steps differ from the correspondingsteps illustrated in FIG. 2, while other steps are like thoseillustrated in FIG. 2.

In steps 2 a, 2 b and 3 c, the MCPTT server includes MSCCK in the SIPmessage request. In step 5, the MCPTT server applies protection(integrity protection, confidentiality protection or both) to the “MapGroup To Bearer” message with the MSCCK as the key. In steps 6 a and 6c, the MCPTT server applies protection (integrity protection,confidentiality protection or both) to the “Map Group To Bearer” messagewith the MSCCK as the key. UEs 405A-C check that the integrity of themessage is genuine (if integrity protection is applied) and decrypt themessage (if confidentiality protection is applied) with MSCCK as thekey.

FIG. 5 is a signal diagram illustrating a method 500 of performingmulticast distribution according to another embodiment of the disclosedsubject matter. In the illustrated example, the method is performed in asystem comprising a plurality of UEs 505A-C, each comprising an MCPTTclient, and an MCPTT server 510, which may be implemented in a networknode, for instance. The method of FIG. 5 is similar to the method ofFIG. 2, with differences that will be apparent from the description thatfollows.

Referring to FIG. 5, the following steps differ from the correspondingsteps illustrated in FIG. 2, while other steps are like thoseillustrated in FIG. 2.

In step 0, UEs that are served by MCPTT server 510 are configured with aparticipating function key (PFK). Subsequently, in step 6, in responseto activation of an MBMS bearer, MCPTT server 510 derives an MBMSsubchannel control key (MSCCK) from the PFK. This key could be madebearer-specific by e.g. using the Temporary Mobile Group Identity (TMGI)as an additional input to the key derivation function.

Similarly, in steps 5 a-c, when UEs 505A-C are informed using unicastSIP signaling about existence of the MBMS bearer, the UEs also derivethe MSCCK from the PFK. In case the key is bound to the TMGI, then theMCPTT client uses the TMGI information received in the notificationmessage to derive the right MSCCK.

When the UE receives a “Unmap Group To Bearer” message or a “Map GroupTo Bearer” message or other MBMS subchannel control messages (e.g. asspecified in 3GPP TS 24.380 subclause 8.4) via an MBMS bearer, e.g. asin step 7, the UE checks the integrity and confidentiality protection ofthe message using the MSCCK, e.g. as in step 8 a or 8 c.

In the method of FIG. 5, it is assumed that MCPTT UEs operating and theMCPTT server within an MCPTT domain are initially provisioned with PFKin step 0. Steps 5 can be executed anytime between steps 3 and 8. If thekey is bound to the MBMS bearer TMGI, then the MCPTT UE use the TMGIreceived in step 3 for the derivation of the bearer-specific MSCCK.

Because currently specified MBMS subchannel control messages can onlycontain one MCPTT Group ID at a time, they are group-specific.Accordingly, an alternative method may be used to accommodate these MBMSsubchannel control messages that are group-specific. The group-specifickey may protect such messages to improve the integrity and/or securityof multicast floor control traffic from the MCPTT server to the MCPTTUEs. When a message for an MCPTT Group containing a MCPTT Group ID(e.g., “x”) is protected by the MKFC of that group “x”, then only themembers of group “x” would be able to decrypt and integrity check themessage.

This alternative, compared to methods 400 and 500, has the potentialbenefit of preventing other MCPTT UEs, belonging to other groups butrunning a session on the same bearer, from decrypting control messagesrelated to other groups. However, because the MKFC is also used inanother session, care must be taken to make sure that there is aseparation mechanism such as using the same index for both sessions.

FIG. 6 is a block diagram illustrating an MCPTT node 600 according to anembodiment of the disclosed subject matter. MCPTT node 600 may be usedto implement e.g. methods such as those illustrated in FIG. 3A, 4 or 5.In some embodiments, for instance, MCPTT node 600 may comprise an MCPTTserver such as MCPTT server 410 or 510.

Referring to FIG. 6, MCPTT node 600 comprises a PTT message generatormodule 605 and a key management module 610. The term “module”, as usedherein, refers to any suitable combination of hardware and/or softwarecapable of implementing the indicated functionality.

PTT message generator module 605 requests an MSCCK to be generated bykey management module 610. Thereafter, PTT message generator module 605generates a key message containing the MSCCK and sends it to UEs using atransmission module (not shown). For further messages, PTT messagegenerator module 610 requests an earlier generated MSCCK and uses theearlier generated MSCCK for hashing and or encryption of the furthermessage.

FIG. 7 is a block diagram illustrating a UE 700 according to anembodiment of the disclosed subject matter. UE 700 may be used toimplement e.g. methods such as those illustrated in FIG. 3B, 4 or 5. Insome embodiments, for instance, UE 700 may take the form of any of UEs405A-C or 505A-C.

Referring to FIG. 7, UE 700 comprises a PTT message handler module 705and a key management module 710. UE 700 receives MCPTT related messagesvia an internal receiver module (not shown). Messages containing keysare transferred to key management module 710. Key management module 710extracts MSCCK's from messages related to the PTT service and storesthem. When PTT message handler 705 receives further messages andrequires a key to check integrity and/or to decrypt, it requests a keyfrom the key management module 710. Key management module 710 providesthe key, and PTT message handler module 705 performs the integrity checkand or decrypt actions with the received MSCCK.

FIG. 8 illustrates a processor-based implementation of a wirelesscommunication network node 800 which may be used for implementing theabove described concepts. For example, the structures as illustrated inFIG. 8 may be used for implementing MCPTT node 600 and/or any of MCPTTservers 410 and 510.

Referring to FIG. 8, node 800 comprises an interface 810 for one or moreradio devices, such as radio device 900 described below. Interface 810may be a radio interface and be used for receiving UL transmissions fromthe radio device(s). Interface 810 may also be used for receiving and/orsending various information, such as for sending keys and encryptedmessages.

Node 800 further comprises one or more processors 850 coupled tointerface 810 and a memory 860 coupled to processor(s) 850. By way ofexample, control interface 810, processor(s) 850, and memory 860 couldbe coupled by one or more internal bus systems of node 800. Memory 860may include a ROM, e.g., a flash ROM, a RAM, e.g., a DRAM or SRAM, amass storage, e.g., a hard disk or solid state disk, or the like. Asillustrated, memory 860 may include software 870, firmware 880, and/orcontrol parameters 890. Memory 860 may include suitably configuredprogram code to be executed by the processor(s) 850 so as to implementthe above-described functionalities of a wireless communication networknode, such as explained in connection with any of FIGS. 3-5.

The structures as illustrated in FIG. 8 are merely schematic, and node800 may include further components which, for the sake of clarity, havenot been illustrated, e.g., further interfaces or processors. Also, itis to be understood that memory 860 may include further program code forimplementing known functionalities of a wireless communication networknode, e.g., known functionalities of a base station or similar accessnode. In some embodiments, a computer program may be provided forimplementing functionalities of node 800, e.g., in the form of aphysical medium storing the program code and/or other data to be storedin memory 860 or by making the program code available for download or bystreaming.

FIG. 9 illustrates a processor-based implementation of a radio device900 which may be used for implementing the above described concepts. Forexample, the structures as illustrated in FIG. 9 may be used forimplementing UE 700 and/or any of UEs 405A-C and 505A-C.

Referring to FIG. 9, radio device 900 comprises a radio interface 910for performing radio transmissions, in particular UL transmissions to awireless communication network. Radio interface 910 may also be used forreceiving and/or sending various information, such as for receiving keysand encrypted messages.

Radio device 900 may further comprise one or more processors 950 coupledto radio interface 910 and a memory 960 coupled to the processor(s) 950.By way of example, radio interface 910, the processor(s) 950, and memory960 could be coupled by one or more internal bus systems of radio device900. Memory 960 may include a Read-Only-Memory (ROM), e.g., a flash ROM,a Random Access Memory (RAM), e.g., a Dynamic RAM (DRAM) or Static RAM(SRAM), a mass storage, e.g., a hard disk or solid state disk, or thelike. As illustrated, memory 960 may include software 970, firmware 980,and/or control parameters 990. Memory 960 may include suitablyconfigured program code to be executed by the processor(s) 950 so as toimplement the above-described functionalities of a radio device, such asthose explained in connection with FIGS. 3-5.

The structures as illustrated in FIG. 9 are merely schematic, and radiodevice 900 may include further components which, for the sake ofclarity, have not been illustrated, e.g., further interfaces orprocessors. Also, it is to be understood that memory 960 may includefurther program code for implementing known functionalities of a radiodevice, e.g., known functionalities of a UE or similar user end device.In some embodiments, also a computer program may be provided forimplementing functionalities of radio device 900, e.g., in the form of aphysical medium storing the program code and/or other data to be storedin memory 960 or by making the program code available for download or bystreaming.

The following is a list of acronyms that may be used in this writtendescription.

-   -   APN Access Point Name    -   ARP Allocation and Retention Priority    -   AS Application Server    -   BM-SC Broadcast Multicast Service Centre    -   CHAP Challenge-Handshake Authentication Protocol    -   CSC Common Services Core    -   CSCF Call Server Control Function    -   CSK Call Service Key    -   D2D Device to Device    -   DL Downlink    -   DPF Direct Provisioning Function    -   ECGI E-UTRAN Cell Global Identifier    -   EPC Evolved Packet Core    -   EPS Evolved Packet System    -   E-UTRAN Evolved Universal Terrestrial Radio Access Network    -   FQDN Fully Qualified Domain Name    -   GBR Guaranteed Bit Rate    -   GCS AS Group Communication Service Application Server    -   GCSE LTE Group Communication Service Enabler over LTE    -   GMK Group Management Key    -   GMK-ID Group Master Key Identifier    -   GMS Group Management Server    -   GRUU Globally Routable User agent URI    -   GUK-ID Group User Key Identifier    -   HLR Home Location Register    -   HSS Home Subscriber Server    -   HTTP Hyper Text Transfer Protocol    -   ICE Interactive Connectivity Establishment    -   I-CSCF Interrogating CSCF    -   IdM Identity Management    -   IdMS Identity Management Server    -   IM CN IP Multimedia Core Network    -   IMPI IP Multimedia Private Identity    -   IMPU IP Multimedia PUblic identity    -   IMS IP Multimedia Subsystem    -   IP Internet Protocol    -   IPEG In-Progress Emergency Group    -   IPEPC In-Progress Emergency Private Call    -   IPIG In-Progress Imminent peril Group    -   KMS Key Management Server    -   MBCP Media Burst Control Protocol    -   MBMS Multimedia Broadcast and Multicast Service    -   MBSFN Multimedia Broadcast Multicast Service Single Frequency        Network    -   MC Mission-Critical    -   MC ID Mission-Critical user identity    -   MCCP Mission-Critical MBMS Subchannel Control Protocol    -   MCPTT Mission-Critical Push-To-Talk    -   MCPTT AS MCPTT Application Server    -   MCPTT group ID MCPTT Group Identity    -   MCPTT ID MCPTT User Identity    -   MEA MCPTT Emergency Alert    -   MEG MCPTT Emergency Group    -   MEGC MCPTT Emergency Group Call    -   MEPC MCPTT Emergency Private Call    -   MEPP MCPTT Emergency Private Priority    -   MES MCPTT Emergency State    -   MIG MCPTT Imminent Peril Group    -   MIGC MCPTT Imminent Peril Group Call    -   MIME Multipurpose Internet Mail Extensions    -   MKFC Multicast Key Floor Control    -   MM Master Key Identifier    -   MONP MCPTT Off-Network Protocol    -   MPEA MCPTT Private Emergency Alert    -   MSCCK MBMS Subchannel Control key    -   NAT Network Address Translation    -   OIDC OpenID Connect    -   PAP Password Authentication Protocol    -   PCC Policy and Charging Control    -   PCK Private Call Key    -   PCK-ID Private Call Key Identifier    -   PCRF Policy and Charging Rules Function    -   P-CSCF Proxy CSCF    -   PFK Participating Function Key.    -   PLMN Public Land Mobile Network    -   ProSe Proximity-based Services    -   PSI Public Service Identity    -   PSK Pre-Shared Key    -   PTT Push-To-Talk    -   QCI QoS Class Identifier    -   QoS Quality of Service    -   RAN Radio Access Network    -   RF Radio Frequency    -   RTCP RTP Control Protocol    -   RTP Real-time Transport Protocol    -   SAI Service Area Identifier    -   S-CSCF Serving CSCF    -   SDF Service Data Flow    -   SDP Session Description Protocol    -   SIP Session Initiation Protocol    -   SRTCP Secure RTCP    -   SRTP Secure RTP    -   SRTP Secure Real-Time Transport Protocol    -   SSL Secure Sockets Layer    -   SSRC Synchronization Source    -   TBCP Talk Burst Control Protocol    -   TEK Traffic-Encrypting Key    -   TGI Temporary MCPTT Group Identity    -   TLS Transport Layer Security    -   TMGI Temporary Mobile Group Identity    -   TrK KMS Transport Key    -   UDC User Data Convergence    -   UDR User Data Repository    -   UE User Equipment    -   UM Unacknowledged Mode    -   URI Uniform Resource Identifier    -   USB Universal Serial Bus    -   WLAN Wireless Local Area Network

While the disclosed subject matter has been presented above withreference to various embodiments, it will be understood that variouschanges in form and details may be made to the described embodimentswithout departing from the overall scope of the disclosed subjectmatter.

1. A user equipment (UE) comprising a Mission-Critical Push-To-Talk(MCPTT) client equipped to be connected to a MCPTT node that serves theUE, comprising: processing circuitry, memory, and transceiver circuitrycollectively configured to: receive a first message from the MCPTT nodein unicast when activating a Multimedia Broadcast and Multicast Service(MBMS) bearer, wherein the first message comprises a MBMS SubchannelControl Key (MSCCK); identify the MSCCK from the message; receive anMBMS subchannel control message from the MCPTT node; and apply integrityprotection and/or confidentiality protection to the MBMS subchannelcontrol message using the MSCCK.
 2. The UE of claim 1, whereindecrypting and/or integrity checking the MBMS subchannel control messageusing the MSCCK comprises generating a message authentication code (MAC)value using the at least one MBMS subchannel control message and theMSCCK, and comparing the generated MAC value to a MAC value appended tothe at least one MBMS subchannel control message.
 3. The UE of claim 1,wherein decrypting and/or integrity checking the MBMS subchannel controlmessage using the MSCCK comprises decrypting the MBMS subchannel controlmessage using the MSCCK.
 4. The UE of claim 1, wherein the at least oneMBMS subchannel control message comprises a Map Group To Bearer message.5. The UE of claim 1, wherein the at least one MBMS subchannel controlmessage comprises an Unmap Group To Bearer message.
 6. A method ofoperating a user equipment (UE) comprising a Mission-CriticalPush-To-Talk (MCPTT) node equipped to be connected to a MCPTT node thatserves the UE, comprising: receiving a first message from the MCPTT nodein unicast when activating a Multimedia Broadcast and Multicast Service(MBMS) bearer, wherein the first message comprises a MBMS SubchannelControl Key (MSCCK); identifying the MSCCK from the message; receivingan MBMS subchannel control message from the MCPTT node; and applyingintegrity protection and/or confidentiality protection to the MBMSsubchannel control message using the MSCCK.
 7. The method of claim 6,wherein decrypting and/or integrity checking the MBMS subchannel controlmessage using the MSCCK comprises generating a message authenticationcode (MAC) value using the at least one MBMS subchannel control messageand the MSCCK, and comparing the generated MAC value to a MAC valueappended to the at least one MBMS subchannel control message.
 8. Themethod of claim 6, wherein decrypting and/or integrity checking the MBMSsubchannel control message using the MSCCK comprises decrypting the MBMSsubchannel control message using the MSCCK.
 9. The method of claim 6,wherein the at least one MBMS subchannel control message comprises a MapGroup To Bearer message.
 10. The method of claim 6, wherein the at leastone MBMS subchannel control message comprises an Unmap Group To Bearermessage.
 11. A method of operating a UE comprising a Mission-CriticalPush-To-Talk (MCPTT) client connected to a MCPTT node that serves theUE, comprising: receiving a first message from the MCPTT node in unicastwhen activating a MBMS bearer, wherein the first message comprises aMultimedia Broadcast and Multicast Service (MBMS) Subchannel Control Key(MSCCK); identifying the MSCCK from the message; receiving an MBMSsubchannel control message from the MCPTT node; and decrypting and/orintegrity checking the MBMS subchannel control message using the MSCCK.12. The method of claim 11, wherein decrypting and/or integrity checkingthe MBMS subchannel control message using the MSCCK comprises generatinga message authentication code (MAC) value using the at least one MBMSsubchannel control message and the MSCCK, and comparing the generatedMAC value to a MAC value appended to the at least one MBMS subchannelcontrol message.
 13. The method of claim 11, wherein decrypting and/orintegrity checking the MBMS subchannel control message using the MSCCKcomprises decrypting the MBMS subchannel control message using theMSCCK.
 14. The method of claim 11, wherein the at least one MBMSsubchannel control message comprises a Map Group To Bearer message. 15.The method of claim 11, wherein the at least one MBMS subchannel controlmessage comprises an Unmap Group To Bearer message.